There are many applications requiring drying of particulate matter. For example, grains and powdered foodstuffs may be dried to prevent decomposition and prolong shelf life, solvent treated pharmaceuticals and other compositions may be dried to remove and/or recover a solvent, and pulped or other processed wastes may be dried to reduce the weight, volume, and objectionable nature of the components to facilitate recovery or disposal.
There are many known methods of drying particulates, including tumble drying, centrifugal drying, flash drying, cyclonic drying, microwave and radiative heat drying. Most of these methods involve some sort of agitation of the material to be dried, along with application of heat. Preferred methods of drying a particular material depend upon many factors, including the wetness of the material to be dried, the tenacity with which the material holds onto the wetting agent, the degree of dryness required, fragility of the material to agitation or heat, the expected disposition of the material when dried, and special difficulties encountered during drying.
Some particulate matter is especially difficult to dry because it tends to aggregate together into masses, cakes and other types of clumps. Such clumps tend to be resistant to drying due to relatively low surface to volume ratios. Increasing the agitation applied to the clumps is not necessarily desirable, as it often merely increases the tendency of such clumps to stick to the inner surfaces of the drying vessel being used. Increasing the applied heat is also not necessarily advantageous, and may lead to incineration or other destruction of the material being dried.
Waste paper pulp provides a good example of these phenomena. Waste paper pulp is generally dried for periods of tens of minutes at 250.degree. F. to 350.degree. F., in a tumble dryer. Attempts at drying paper pulp using rapid movement or compression of the pulp only leads to further aggregation of the material, which in turn causes even greater difficulties. Increasing the applied heat is helpful up to a point, but eventually leads to burning of the pulp fiber ends. In addition to all of these other difficulties, known methods for drying waste pulp generally result in relatively large clumps, averaging several millimeters across. Such clumps have little or no commercial value for making paper because of their low degree of fluffiness, and are usually discarded. Fluff generators are known, but add additional expense and still do not necessarily provide adequate results due to fiber damage. Other particulate materials besides waste paper pulp have similar difficulties, including various natural and artificial fibers wool, cotton, and polyester.
There are a few technologies which provide a very high degree of agitation at a relatively low temperature. U.S. Pat. No. 5,548,905 to Kuma et al. (August 1996), describes a rapid dehydrating and drying method in which a high speed, low temperature air stream (approximately 60.degree. C.) is applied against sheet-like articles such as mats, carpets, and fabrics. In that patent the drying is effected by a strong negative pressure air stream used alone or in combination with a strong positive pressure air stream. The Kuma et al. method, however, is not suitable for particulate matter because there is no provision for immobilizing the particles against the high speed air stream. U.S. Pat. No. 4,695,248 to Gray et al., describes a method of drying particulate matter using very high turbulence provided by the exhaust of a pulse jet engine. In that patent a typical temperature is recited at 2500.degree. F., although a very short residence time on the order of milliseconds of the material being dried is reported to result in dried solids at about 100.degree. F. to 150.degree. F. The Gray method is, however, is problematic in that it damages the ends of fibers, is relatively expensive to implement, and may be impractical in many circumstances.
Drying of particulates in cyclonic dryers is known to have several advantages, including cost effectiveness and applicability to many different materials. Cyclonic dryers, however, are not generally known to be suitable for materials which tend to aggregate into clumps when damp. U.S. Pat. No. 4,057,908 to Mirliss et al. (November 1977), for example, describes the use of a cyclonic dryer for drying damp powders, but the cyclonic dryer is used in combination with a pre-dryer where the damp material is suspended on a column of low velocity heated gas until it is sufficiently dried to enter the cyclonic portion of the dryer.
In FIG. 1 a typical prior art cyclone dryer 10 generally has an upper section 20, a middle section 30, a lower section 40, an inlet 50, an upper outlet 60 and a lower outlet 70. The cyclone dryer 10 is largely hollow, with the upper, middle and lower sections 20, 30, 40 cooperating to provide a substantially continuous cavity 80. Cyclone dryer 10 is operated by introducing an air stream depicted by arrow 90 into the inlet section 40, and directing the air stream 90 along the periphery of the cavity 80. The air stream 90 generally flows in a downward spiral, substantially tangential to the longitudinal axis of the cavity 80, and exits the dryer 10 at lower outlet 70. Such motion produces un upward flowing vortex 96, which exits the dryer 10 at upper outlet 60. When conditions are appropriate, a material to be dried is introduced into the dryer along with air stream 90, carried along the downward spiral 96, back up in vortex 96, and finally carrier out of the dryer at upper outlet 60.
Typical transit time of particulates in a cyclone dryer such as that depicted in FIG. 1 is about 3 to 5 seconds. Transit time is known to be affected by several variables, including the dimensions of the cavity, velocity of the air stream within the cavity, and density and other flight characteristics of the damp material. Drying is of the material is also known to be affected by several variables, including the amount of moisture or other solvent in the material, the surface area and other characteristics of the material, the transit time of the material in the dryer, and the temperature and moisture carrying capacity of the air stream. Cyclonic dryers are typically operated using an inlet air stream velocity of between 25 and 100 feet per second, and an inlet air stream temperature of between 100.degree. F. and 500.degree. F. These and other details are described in standard reference works, including the Handbook of Industrial Drying. (2.sup.nd Ed., Arun S. Mujumder, editor, 1995).
Materials such as waste paper pulp carrying more than about 60% weight of water or other solvent cannot effectively be dried in a prior art cyclone dryer such as that shown in FIG. 1. Years of experimentation have demonstrated that this is true regardless of variations in the size and shape of the various sections, and the velocity and temperature of the air stream. The main problem is that relatively low velocity air streams (25 and 100 eet per second) are insufficiently turbulent and provide insufficient transit time to dry the damp material, while relatively high velocity air streams (100 and 500 feet per second) impart such centrifugal force to the material that it tends to the side walls of the dryer. This dichotomy has been partially addressed in the art. U.S. Pat. No. 4,089,119 to Heinze (May 1978) and U.S. Pat. No. 5,333,392 to Heinze (August 1994) describe dryers containing annular baffles which alter the air stream such that a high velocity (20 m/sec to 100 m/sec) and short residence time (on the order of a few seconds) prevail in the lower part of the dryer, and a low velocity and high residence time prevail in the upper part of the dryer. Additionally, U.S. Pat. No. 5,647,142 to Anderson (July 1997) describes the inclusion of perforated plates to increase transit times. Even these solutions, however, are only partially satisfactory because they still require the air stream to be heated to effect substantial drying.
Thus, there is a continued need for methods and apparatus which provide low temperature drying of particulates, especially particulates which have a tendency to aggregate into a mass when damp.